Among the questions to be addressed by a Mars Sample Return Mission are the history of the Martian climate and the mechanisms that control the volatile cycles. Unfortunately, the evidence that bears most strongly on those issues lies in the volatile distribution in, and physical configuration of, a very delicate and volatile system: the uppermost Martian regolith. Some useful measurements to be made on returned samples of the regolith are identified, along with the many critical considerations in ensuring the usefulness of returned samples

Zent, A. P.

English

NASA Technical Reports Server

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N89-18370 
184 
THE BECOIU) OF MARTIAN CLIMATIC HISTORY IN CORES AND ITS PBESERVATION 
Zent, A. P., Planetary Geosciences Division, Hawaii hstitute of Geophysics, University of Hawaii, Honolulu, 
Hi. 96822 
Among the questions to be addressed by a Mars Sample Return Mission are the history of the martian 
climate and the mechanisms that control the volatile cycles. Unfortunately, the evidence that bears most 
strongly on those issues lies in the volatile distribution in, and physical configuration of, a very delicate and 
volatile system: the uppermost martian regolith. This study identifies some useful measurements to be made 
on returned samples of the regolith, and a few of the many critical considerations in ensuring the usefulness 
of returned samples. 
Among the moet difficult properties of the regolith to examine via returned sample are: the vertical dis- 
tribution of ground ice, the vertical distribution of adsorbed H20, and the amount of adsorbed C02. 
The ability of the martian regolith to transport H a 0  is very limited; only ,. 1 - 10 g HZ0 cm-’ can 
exchange with the regolith per obliquity cycle. Therefore the abundance and distribution of Ha0 through 
the uppermost regolith is the product of an extensive history of HzO migration. testing for the presence or 
absence of ground ice in the shallow, high-latitude regolith would provide a measure of our fundamental 
understanding of the martian H20 cycle. If present, it would confirm our model of the high-latitude regolith 
as a sink for H20. The depth at which ground ice is found could be related to the thermal properties of the 
regolith, which control propagation of heat to the ice, and to the long-term average atmospheric partial pres- 
sure of H20.  If absent, it would cause a re-examination of our models, and could lead to constraints on the 
erosion/deposition rates of the regolith. 
Whether there is ground ice present or not, there is certain to be a finite amount of adsorbed H20.  
Diurnal, seasonal and long-term cycles of temperature and P, act to modify the abundance and distribu- 
tion of the adsorbate. Because the depth to which each cycle acts is proportional to the square root of the 
cycle’s period, the depth profile of H 2 0  adsorbate may be invertible to yield information on the history of the 
H 2 0  cycle. Further laboratory and numerical work is necessary to develop the possibility. 
The largest portion of exchangeable C02 in the martian environment is physically adsorbed onto the 
regolith. Models suggest that at high obliquity, the regolith desorbs enough C02 to raise the atmospheric 
pressure by several millibars. We would like to measure the amount of COZ available for exchange with the 
atmosphere. One concern is the competition for adsorption sites by H2O vapor. Another is the degree to 
which C02 is chemisorbed onto the particulate material, and how that might alter the soil’s surface func- 
tional groups. For example, the difference between chemisorption onto inorganic hydroxyl groups and simple 
outer-sphere complexation on siloxane ditrigonal cavities woqld be substantial in terms of adsorptive 
exchange. 
The interaction of the regolith and atmosphere is also controlled by the small-scale structure of the 
regolith. For example, the thermal conductivity of the regolith can depend upon the geometry of packing, the 
nature of grain boundaries, and the presence of intragrain bonding, which might allow conduction between 
grains to compete with gaseous conductivity. The pore sise, in relation to the mean free path of a gas 
molecule determines the gas-phase conductivity, especially important in loose, unbonded material. 
The pore sise distribution determines the efficacy of volatile transport through the regolith, and the 
capacity of the regolith to buffer climate changea. If possible, we would like to measure the pore size distri- 
bution in a number of relatively undisturbed samples. 
We conclude that there is a great deal to be learned from examination of the uppermost martian regol- 
ith if the chemical state and structure could be preserved. 
Cores are the optimal method for retrieving and examining the material in the shallow martian regol- 
ith. A great deal of work has been done on core acquisition, although several aspects bear additional 
emphasis. One of the first is the extreme importance of using a minimum penetration rate, which is propor- 
tional to the temperature of the core at the bit during drilling. Because of the extreme volatility of physi- 
cally adsorbed COz, the core must be kept at the minimum ambient temperature. The drill stem should be 
constructed of low thermal conductivity material to avoid transporting heat along the stem during drilling 
operations. 
https://ntrs.nasa.gov/search.jsp?R=19890008999 2020-03-20T04:06:42+00:00Z
Cl&ate and Cores 
Zent, A. P. 
I 
185 
Preservation of the cores after drilling is must also be addressed. There should be rapid transfer from 
the drill stem to the storage container. The seasonal thermal profile is such that the temperature at the sur- 
face is different from the temperature at depth. Since the cores are to  be no more than 1 - 2 cm in diameter, 
they will approach the surface temperature, which may differ from their original temperature by > 40 K, in a 
matter of minutes. There is the possibility of escape,of gas at the margins of the core, and for migration of 
gas in response to the changing temperature profile. If possible, the transfer from core stem to storage should 
take place at night, although this will place constraints on our ability to inspect the core and to make go- 
nogo decisions on storage. In any event, the leisurely examination of the core in the full sunlight will cause 
an unacceptable loss of information, and should be absolutely disallowed. 
The typical lifetime of the cores after extraction, and before examination, will be several hundred days, 
during which time significant alteration is possible. The optimal solution to this problem is to return sub- 
samples of taken at predetermined intervals along core. The subsamples should be stored and transported 
separately. In addition to preserving the gradients, albeit at degraded resolution, the subsampling technique 
would allow more sample to be returned, since returning the entire core is probably wasteful of mass. Our 
calculations suggest that the uncertainty in average PH* associated with a 5 em uncertainty in the position 
of the interface is only "10%. A thud reason for the return of s m d  subsamples of the cores is the impor- 
tance of preserving the pore size distribution of the regolith. Smaller samples place less load on contact 
points during acceleration, and are less likely to result in result in compaction. 
If it proves prohibitively expensive or complex to  implement adequate techniques for the acquisition 
and subsectioning of contiguous cores, then thought should be given to abandoning the coring tool alto- 
gether, and simply returning samples dredged from a variety of depths in adjacent excavations. In the 
absence of local variations in surface units, the volatile distribution in the subsurface should be sufficiently 
homogeneous to make all locations equally representative. 
The samples should be continuously monitored after excavation, and provision should be made for cool- 
ing of the sample container should the temperature rise substantially above the temperature at the time of 
collection. 
Finally, there should be no sterilization whatsoever on the core samples to be analyzed for the state 
and distribution of major volatiles. Even heating samples to room temperature is likely to result in wholesale 
chemical and physical redistribution of volatile species. 
Cores should be collected from two well separated latitudes on the surface of the planet. The goal in so 
doing would be to establish a the latitudinal distribution in the volatile abundance8 of the regolith, both as 
an historical indicator of the past climate, and as an aid in understanding how the martian climate will 
respond to future insolation variations. The cores should be at least 1 m in depth, in order to sample volatile 
distributions that are indicative of both annual and longer term averages. Since only subsamples of the cores 
are logical for return, the option of drilling to 2 m might be explored, depending on the latitude at which 
drilling is to take place. One core should be from latitudes poleward of 40') in order to sample the hard- 
frozen permafrost. A second lander/rover should be included in the mission for redundancy and targeted for 
the low-latitude regolith. 
With these criteria and others, we have selected two options for landing sites for a lander-rover tan- 
' dem. The additional criteria we have considered are as follows: Both landing sites should be in geologically 
complex areas, with the opportunity to visit a maximum number of terrains with total traverses of no more 
than 100 km; at least one of the rovers should be able to sample volcanic terrains. Our high-latitude site is 
at 316'W - 50'30' N. It is very near the boundary of the northern plains and cratered highlands. The rover 
should be able to example the stratigraphy of the boundary. The second site is at 61"30'N - 0'30'N. This 
site is in the ridged plains material, and very near the channel system that runs north from Juventae 
Chasma. The channel system forma a geologic boundary between the volcanic terrains on the west, and the 
heavily cratered terrain on the east. 


The record of Martian climatic history in cores and its preservation

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